Various salts of 1,2-ethylenebisdithiocarbamic acid have been known for many years as agents for combating plant diseases caused by fungi. Among the ethylenebisdithiocarbamates (sometimes referred to as "EBDC") useful as plant fungicides are the manganese, zinc, nickel, cobalt, copper, sodium, potassium and ammonium salts of 1,2-ethylenebisdithiocarbamic acid or co-reacted metal EBDC. Preferred fungicides of this class are manganese EBDC (maneb), zinc EBDC (zineb), and particularly zinc coordination complexes of manganese EBDC (mancozeb).
A problem with the EBDCs is that they tend to degrade over time due to factors including oxidation, heat, humidity, etc., into, among other things, ethylenethiourea (2-imidazolidinethion), commonly known as ETU. Due to this degradation, ETU content increases in concentration during storage of the EBDC. Since ETU has been found to have carcinogenic and teratogenic effects in laboratory animals, and no significant biological activity as a fungicide has been observed, ETU is an unwanted degradation product.
Over the years a number of processes and additives have been developed to reduce the ETU content of EBDCs. It is desirable that the ETU content of EBDC formulations be reduced to less than 0.015 percent by weight, based on the weight of the EBDC in the formulation. Several prior attempts have been made to reduce the content of ETU in EBDC by adding formaldehyde or formaldehyde precursor (formaldehyde donor) to the aqueous reaction mixture, preferably with a water-soluble zinc salt, and also by optionally adding paraformaldehyde or another formaldehyde releasing agent (precursor or generator) to the dried product. See, for example, U.S. Pat. Nos. 4,217,293 and 4,344,890 of Adams. However, processes of even further reducing ETU content in EBDC are desired.